The present invention relates to Digital Video Broadcasting-Satellite-Second Generation (DVB-S2) standard for satellite communication systems, in particular, Adaptive Coding and Modulation(ACM) mode for DVB-S2.
ACM enables satellite service providers to manage their networks with much more flexibility by virtue of its continual adaptation to link conditions to each and every remote, resulting in improved bandwidth utilization. DVB-S2 incorporates the use of variable modulation types used to accomplish maximum benefit over a radio link. Careful link operating point selection is needed to optimize the performance of the selected modulation and maximize the throughput and performance.
DVB-S2 standard supports mainly four modulation types: Quadrature Phase Shift Keying (QPSK), 8 Phase Shift Keying (8PSK), 16 Asymmetric Phase Shift Keying (16APSK) and 32 Asymmetric Phase Shift Keying (32APSK). QPSK and 8PSK are proposed for broadcast applications, and can be used in non-linear transmitters driven near to saturation. 16PSK and 32PSK are used mainly for professional, semi-linear applications but can also be used for broadcasting under certain conditions.
In a satellite communication system, each transmitter will be processing a very large number of messages simultaneously. A transmitter may transmit a maximum signal strength when operating at a saturation output power level. However, operating at saturation will increase non-linearities in the transmitter amplifier. Any non-linearity in the transmitter amplifier may lead to intermodulation, which causes interference between the message signals by transferring modulations from one frequency range to another. One of the ways to reduce intermodulation is by using output back-off (OBO). OBO is the amount (in dB) by which the output power level of the amplifier is reduced, or “backed-off,” from the saturation output power level.
FIG. 1 illustrates a typical physical layer signal for DVB-S2 broken in to a sequence of frames 100.
As shown in the figure, sequence 100 includes a packet 102, a packet 104, a packet 106, a packet 108, a packet 110 and a packet 112. Each packet includes a header with a frame.
The header associated with each frame may contain synchronization and signaling information. Within each frame, the coding and modulation type is homogeneous, but may change due to ACM in adjacent frames. As an example, packets 102 and 110 were created with a QPSK modulation type, packets 104 and 106 were created with a 16APSK modulation type, packet 108 was created with a 32APSK modulation type and packet 112 was created with an 8PSK modulation type. In this example, term packet is used but sequence 100 could be bursts or code blocks of data.
For any linear transmitter, it is desirable to transmit at the highest power level possible without affecting the signal. When transmitting using modulation types like QPSK and 8PSK (single ring constellations), the power level required to operate a transmitter stays the same. Relative to QPSK and 8PSK, for a transmitter to transmit with higher modulation types likes 16APSK and 32 APSK, the transmitter will require different OBO power levels. Higher modulation types have constellations with multiple power levels, where each power level is best associated with its own OBO. If a transmitter is operable to transmit packets having a plurality of different modulations modes, the transmitter must operate at the largest OBO. For example, for purposes of explanation, presume that the OBO associated with a 16APSK modulation type is larger than the OBO associated with an 8PSK modulation type. To be able to transmit packets for both the 8PSK and the 16APSK modulation types, a transmitter must reduce its overall operating level to the OBO of the 16APSK. However, in such a situation, the transmitter will not optimally transmit the packets that have been modulated with the 8PSK modulation type because the power level has been backed-off more than OBO associated with the 8PSK modulation type. In other words, to reduce the overall power for all modulation types, the power level for lower modulation types are required to be reduced as well, which may adversely affect the throughput of the transmitter.
It can be shown that QPSK and 8PSK modulation types have similar operating point back-off in the case of a satellite link or transmitter operating point in terms of OBO. But higher order modulation rates can be shown to require a higher OBO to accomplish optimum link performance due to the higher peak to average ratio of the modulation. As an example, where QPSK and 8PSK may need to operate at 0.5 dB OBO, 16APSK may need to be run another 1 dB (1.5 dB OBO) to achieve the desired link performance.
What is needed is a method to improve the throughput for higher modulation types without affecting the lower modulation types using the same transmitter in satellite communication systems.